TH425 issue , not shifting gears (77 Eldo)

Hi @ all. Yesterday I noticed a big problem with my Eldorado (1977 FWD). After driving for aprox. 35min I parked the car to get some groceries.

When I came back I started the engine , put in "DRIVE" and, as usual, waited for the tranni to shift in the gear. but then I noticed engine power was not delivered to the wheels.

Put in "reverse", wait for the shift, and the car drove backwards normally. Put in "DRIVE" again , same problem. Put in Park, waited to shift, put in "low" - waited to shift and the car moved slightly forward. pressed the accelerator firmly , still not delivering engine power to the wheels, step off the accelerator and a loud crank - sound appeared. Then the gear was in and the car moved .

Put in "DRIVE" and I could hear the tranni shifting and the car drove, but it dodnīt upshift anymore. So I took the car home in what i assume was first gear.

At home I cheked the vacuum line for the modulator and that seems to be fine, checked the down-shift switch near the carb with an ohmmeter and that is fine,too. The filter and oil was changed 4 month ago.

Any ideas what I can check furthermore while the tranni is inside the car ?

Is it possible for "normal" people to pull the tranni out ? As fas as i understand the service manual there are no special tools required, but the tranny looks pretty big and seems like a pain in the .... to be pulled out.

About how far do i need to raise the front of the car to get the tranni out and how much is aprox. the weight of the tranni?

Re: TH425 issue , not shifting gears (77 Eldo)

If they still had a vacuum modulator in 76 that would hold the shift until the mecahical governor flyweights shift it for you at much higher rpm. it may not shift out of 1st gear til' you hit 35- 40mph. These modulators don't usually fail their function but will occasially rupture a diaphram & stuck ATF directly into the intake. If you took the car out and allowed it to rev up it make shift it will be very hard. First look for a vacuum hose/line leak.

The common failure is a vaccuum leak down where the vacuum line connects to the modulator valve. If not there go backwards up the line that is usually steel. I have also seen the steel line rubbing on something that creates a hole right through. That car may have a junction that gangs several lines.

The hose in this pic is the most common source of a leak. It couples with the steel line. Usually leaks because the rubber hardened & cracked or oil soaked & swollen too big. The same can happen at the intake if it has a dedicated port, It may be directly behind the carb.

Re: TH425 issue , not shifting gears (77 Eldo)

Hi,

yes the 77 use a "standard" TH425 transmission with a vacuum modulator.

Hmm I replaced all rubber lines two years ago because, as you mentioned, they were all cracked or broken. The engine runs really well and it took me a lot of time until I had the engine rebuild and now the damn tranni fails.

Thatīs a pic when I was almost done with all the vac-lines.

. I īll replace the whole line (metal and rubber) against a new hose tomorrow and will take the car for a test drive and let you know if that was the cause, but I doubt that the vacuum line is causing the trouble.

Re: TH425 issue , not shifting gears (77 Eldo)

I replaced the modulator today, still no luck. I adjusted the modulator (clockwise - 2 turns of a 4 turn maximum) as described but no luck again. When I pulled the old modulator about a gallon of ATF was coming out of the transmission. The oil is less than a year old, I wonder that it is all black. I always in automatic transmissions the oil can be purged after 100000 miles and it is still clear and red ? I assume something got burned inside the tranny I also pulled the rod that is behind the mdoulator and it looks fine, no scratched surfaces.

I also got a governor replacement kit , but I assume now that somehow the clutches are fried. But what i wonder ist that the reverse gear always goes in correctly , only the forward gears seem to not transfer engine power. Do I need to pull the transmission now ?

Re: TH425 issue , not shifting gears (77 Eldo)

I replaced the modulator today, still no luck. I adjusted the modulator (clockwise - 2 turns of a 4 turn maximum) as described but no luck again. When I pulled the old modulator about a gallon of ATF was coming out of the transmission. The oil is less than a year old, I wonder that it is all black. I always thought that in automatic transmissions the oil can be purged after 100000 miles and it is still clear and red ? I assume something got burned inside the tranny I also pulled the rod that is behind the mdoulator and it looks fine, no scratched surfaces.

I also got a governor replacement kit , but I assume now that somehow the clutches are fried. But what i wonder ist that the reverse gear always goes in correctly , only the forward gears seem to not transfer engine power. Do I need to pull the transmission now ?

Re: TH425 issue , not shifting gears (77 Eldo)

I re read your 1st post. You say you could feel it shift but then you say it seemed to stay in low gear? confused? If you shift it manually from L to 2 to 3 does it shift? The linkage from the steering column is secure, no bushings missing? By 'loud crank" do you mean a clank noise or metal snapping noise?

Re: TH425 issue , not shifting gears (77 Eldo)

Yes I asked Dad to shift while I was checking the linkage and the linkage is always moving correctly.

What I meant in my first post was that regardless of which forward gear (drive or Low setting) the engine power is not transmitted to the wheels. When I first encountered that problem at the parking lot I probably moved the lever 10-20 times from Low to Drive and back until the transmission made a real loud metallic sounding clock noise , similar to slamming a door. Then engine power was transmitted and I took the car home.

You can hear a sound that the transmission is doing something when I put in Drive or Low , but it does not really put a forward gear in. When I put in reverse after a second the transmission makes this sound also but in reverse it is transmitting engine power to the wheels while in Drive or Low no power is transmitted.

I studied the service manual and they say first thing to be checked should be oil pressures in certain circumstances with an oil pressure meter. Not sure if that will really help me and Iīd need to get a pressure tester,too.

Re: TH425 issue , not shifting gears (77 Eldo)

My 1st thought is a failed drive axle, not a transmission. If not an axle then the differential ring gear or spider gears. With the front of the car up safely on jack stands, I would check both drive axles for torn boots or play in each inner and outer tripot joint. If both axles appear to have no play or failed boots, I would remove the tin cover on the differential and check the ring gear and spider gears.

Re: TH425 issue , not shifting gears (77 Eldo)

Hi Stephan (sdoom),
I spotted your thread on the Cadillac forum.
Did you manage to get your car fixed as I appear to have the exact same problem with the forward gears on my 1977 Cadillac Seville ?
I have checked the oil level which appears ok.
I can engage reverse but not the forward gears.
The once that it did appear to engage a forward gear I went up the road and the car would not change up through the gears.
The way you described your problem seems exactly the same as mine.
Can you help please ?
Tony Winters.

Re: TH425 issue , not shifting gears (77 Eldo)

Thanks for your reply regarding my Cadillac Seville.
I will check the items you mention.
I think my trans. is a 3-speed Turbo Hydra-Matic 375, which I understand is a smaller version of the 400 trans - not totally sure.
Will let you know how I get on.
Best Regards,
Tony Winters.